Solution: If boys want to have X people, girls have 2x/3 people (the ratio of boys to girls is 3: 2, which is used to set an unknown number).

X÷(2x/3 +6)=5/4 (after six girls joined, the ratio of male to female was 5: 4, which was used to make the equation).

X=45，2x/3=30。

A: There used to be 45 boys and 30 girls.